As many have pointed out on this forum, these various timescales do have very specific meanings which often fade at levels coarser than a few nanoseconds (modulo 1 second), and which at times are misapplied at the 1-second and higher level.
GPS Time is technically an "implicit ensemble mean". You can say it exists inside the Kalman Filter at the GPS Master Control Station as a linear combination of corrected clock states. But there is no need for the control computer to actually compute it as a specific number, and that's why it is implicit. Every GPS clock is a realization of GPS Time once the receiver applies the broadcast corrections. GPS Time is steered to UTC(USNO), and generally stays within a few nanoseconds of it, modulo 1 second. UTC(USNO) approximates UTC, and so it goes. The most beautiful reference to GPS Time is "The Theory of the GPS Composite Clock" by Brown, in the Proceedings of the Institute of Navigation's 1991 ION-GPS meeting. But others, including me, routinely publish plots of it. --Original Message----- From: Leap Seconds Issues [mailto:[EMAIL PROTECTED] On Behalf Of Ashley Yakeley Sent: Tuesday, January 09, 2007 2:22 AM To: LEAPSECS@ROM.USNO.NAVY.MIL Subject: Re: [LEAPSECS] Introduction of long term scheduling On Jan 8, 2007, at 22:57, Steve Allen wrote: > GPS is not (TAI - 19) What is GPS time, anyway? I had assumed someone had simply defined GPS to be TAI - 19, and made the goal of the satellites to approximate GPS time, i.e. that GPS and TAI are the same (up to isomorphism in some "category of measurements"). But apparently not? Are the satellite clocks allowed to drift, or do they get corrected? -- Ashley Yakeley